The local anaesthesia most commonly used in the upper jaw is a nerve plexus method. It consists of a supraperiosteal injection administered into the buccal side of the tooth being treated, near to the apex of the root. The procedure is simple, rapid and shows satisfactory efficacy and duration. However, it is inconvenient if anaesthesia is required for more than one dental element, a situation which would naturally entail a series of repetitions of the procedure.
The anatomy of the upper jaw’s nervous system allows the performance of anaesthesia at the level of the nerve trunk. The most well-known techniques, however, have the disadvantage of affecting large areas of the lips and face. One example which could be mentioned is anaesthesia of the infraorbital nerve, an effective method, but one which entails a certain amount of postoperative discomfort for the patient.
Origin and effects of the AMSA technique
In 1998 Friedman and Hochman introduced an anaesthesiology technique capable of anaesthetising the central and lateral incisor, the canine and first and second premolars and the soft tissues of the palate (without involvement of the lips and face), all with a single administration.
Because of the advantages mentioned above, the method is indicated in conservative interventions. In addition, thanks to its excellent hemostatic action on soft palate tissues, it is also used in periodontal surgery.
The technique became known as AMSA, since it involves the anterior superior alveolar nerve (ASA) and the middle superior alveolar nerve (MSA), both collateral branches of the aforementioned infraorbital nerve originating within the homonymous canal. The two nerves originate 5–8 mm and 10 mm, respectively, posterior to the infraorbital foramen. The first carries nerves from the pulp of the two incisors and the canine, the second those of the two premolars and the mesial-buccal root of the maxillary first molar. It should be noted that the MSA nerve is in fact not a constant structure: anatomical studies on cadavers detect its presence in between 30 and 72% of individuals. In its absence, there will be a compensatory plexus between the ASA and the superior posterior alveolar nerve.
The biological rationale of the technique is the diffusion — obtainable through the application of controlled pressure — of the anaesthetic solution through the nutrient vessels and the cortical porosities in the palate.
The insertion site is located in the palate on the bisection of the first and second premolars, halfway between the middle palatine raphe and the crest of the free gingival margin. This corresponds to the area of confluence of the ASA and MSA nerves.